Public safety radio systems have been widely used by police and fire departments for decades. Such systems are typically based upon a one-point to many-points format wherein a transmission by one radio is heard by a number of other radios. Signals are often transmitted and received over a single radio frequency carrier in such systems.
Public service radios often use frequency division multiple access (FDMA) for communications. Under such a system, individual radios transmit a request to talk to a zone controller. In response, the zone controller may return an authorization along with an identifier of the carrier to transmit on.
Individual users are often included within specific talk groups and a transmission often includes an identifier of the user and/or talk group. The zone controller identifies the talk group and routes the transmission appropriately.
In order to reduce congestion, public safety radio systems also use time division multiple access (TDMA) radio carriers. However, many existing radios only exchange signals under a FDMA format and, as a result, TDMA carriers have had to coexist side-by-side with FDMA carriers. However, the maintenance of separate carriers is inefficient and a need consequently exists for a method of sharing carriers.